A conventional oscillometric sphygmomanometer detects the oscillation of the cuff pressure based on the volume change of the artery positioned below an occluding cuff while gradually raising the pressure of the occluding cuff to a pressure higher than the systolic blood pressure or decreasing the pressure of the occluding cuff from a pressure higher than the systolic blood pressure, and determines the systolic and diastolic blood pressure by the amplitude change of the oscillation.
The blood pressure measuring method using an occluding cuff as described above obtains the systolic blood pressure by detecting a phenomenon in which the blood flow in the artery stops flowing blood when the pressure of the occluding cuff is raised to be higher than the systolic blood pressure as a maximal pressure in the artery, and starts flowing blood when the pressure of the occluding cuff is lowered.
A presently widely used Korotkoff method (auscultatory method) obtains a systolic blood pressure value (maximal blood pressure value) by stopping the blood flow once by making the pressure of an occluding cuff higher than the systolic blood pressure, gradually decreasing the pressure of the occluding cuff, and detecting, on the downstream side of the occluding cuff, Korotkoff sounds generated at the timing at which the blood starts flowing again.
The above-mentioned oscillometric method is a method of detecting the phenomenon in which the blood starts flowing again as the pressure oscillation of an occluding cuff based on the volume change of the artery below the occluding cuff. When compared to the Korotkoff method, therefore, the oscillometric method has the advantage that the manufacturing cost can be cutting because (a stethoscope including) a sensor for sensing Korotkoff sounds is unnecessary.
The auscultatory method has the drawback that noise (scratch noise and vibrations of cuff cloth and cuff tubes) generated during blood pressure measurement is readily detected by mistake because the frequency component of the above-mentioned noise is close to that of Korotkoff sounds. On the contrary, the frequency component of the pressure oscillation is lower than that of above-mentioned noise when measurement is performed by the oscillometric method. Also, this frequency component largely differs from the noise frequency generated during blood pressure measurement, so the oscillometric method is not influenced by the noise. In addition, the oscillometric method can perform measurement even when a cuff is attached to a position deviated from the artery as a portion to be measured. Accordingly, the oscillometric method is mainly used for an automatic sphygmomanometer.
When measuring the systolic blood pressure, however, the oscillometric method has a problem caused by the blood vessel compressing characteristic of a Riva-Rocci cuff used as an occluding cuff. That is, a compressing force reflecting the cuff pressure faithfully can be obtained in a middle portion in the widthwise direction of the Riva-Rocci cuff, but a compressing force reflecting the cuff pressure faithfully can not be obtained in a portion deviated from the middle portion. The compressing force gradually reduces from the middle portion toward the end portions of the cuff and becomes zero in the end portions.
By the characteristic described above, when the cuff pressure of the occluding cuff is close to and slightly higher than the systolic blood pressure at the timing immediately before the measurement of the systolic blood pressure, the blood flow is stopped in the middle portion of the cuff. Consequently, in synchronism with the heart beat, the blood flows from the upstream portion of the occluding cuff to its middle portion and returns. By this phenomenon, a pulse wave is already detected before the generation of a pulse wave used to detect a phenomenon, as a detection target of the systolic blood pressure, in which the blood starts flowing again toward the downstream side (forearm side) of the cuff.
Also, when the cuff pressure of the occluding cuff becomes lower than the systolic blood pressure and the blood starts flowing again, this blood flow causes a volume (of artery) change downstream of the middle portion under the occluding cuff. Since the pressure of the occluding cuff is slightly lower than the arterial pressure when this volume change occurs, the blood vessel closes immediately after it has opened for a very short time. In this state, the volume change on the downstream side (part from center to downstream edge) of the occluding cuff is much smaller than that on the upstream side (part from center to upstream edge).
A pulse wave detected by the oscillometric method is based on a volume change obtained when the volume changes on the upstream and downstream sides of the occluding cuff described above are superposed. This makes it very difficult to selectively detect only the change based on the restart of the blood flow by the pulse wave especially when the blood flow amount is small. The foregoing is the cause of making the S/N ratio of systolic blood pressure measurement of the oscillometric method lower than that of the Korotkoff method.
When the cuff pressure is further lowered from the systolic blood pressure, a time during which the arterial pressure is higher than the cuff pressure gradually prolongs within one heart beat period. Since this increases the volume change on the downstream side of the cuff, the amplitude of the pulse wave gradually increases. Furthermore, although it depends on the degree of congestion of blood on the downstream side, when the internal pressure of the blood vessel in a peripheral portion of the artery from the cuff becomes higher than the cuff pressure, a reflection pressure phenomenon occurs from the periphery. This reflection abruptly increases the pulse wave.
When the cuff pressure further decreases, the time during which the internal pressure of the blood vessel in the peripheral portion from the cuff is higher than the internal pressure of the cuff prolongs. In addition, immediately before the time during which the blood vessel closes within one oscillation period disappears, the blood vessels in the upstream and downstream portions of the cuff simultaneously fully open to maximize the amplitude of the pulse wave.
In the systolic blood pressure measurement by the oscillometric method, the volume (of artery) change under the cuff at the timing of the systolic blood pressure measurement is mainly the change on the upstream side of the cuff central portion, which is equivalent to about 50% of the overall blood vessel volume under the cuff. Therefore, the timing at which the pulse wave amplitude is about 50% of a maximal pulse wave amplitude produced when almost the entire blood vessel below the cuff repeats full open and full closure is used as the systolic blood pressure.
Unfortunately, this ratio is influenced by that unbalance of the volumes in the upstream and downstream portions which is caused by the way the cuff is wrapped and contributes to the formation of the pulse wave under the cuff, a compliance difference produced by the strength with which the cuff is wrapped, the magnitude of the rise in internal pressure of the blood vessel in a peripheral portion, and the change rate. Also, the rise in internal pressure of the blood vessel in a peripheral portion is influenced by the degree of congestion of blood resulting from a short repetitive time of blood pressure measurement, but mainly influenced by the blood pressure value, the degree of peripheral circulation, and the peripheral blood vessel compliance, as differences between individual living bodies.
To solve the above problems, a triple-cuff method has been proposed as a technique in which a sub air bag is formed between an occluding air bag and measurement portion and on the upstream side (a side close to the heart when a cuff is attached to a limb) of the occluding bag, the difference between the measurement portion compressing forces of a middle portion and upstream end portion in the thrust direction of the occluding air bag (upstream end portion compressing force<middle portion compressing force) is decreased, and the entrance of a pulse wave on the upstream side is decreased when the pressure of the occluding air bag is slightly higher than the systolic blood pressure, thereby decreasing the influence of the pulse wave on the upstream side.